1. Technical Field
The present invention relates to a magnetic resonance imaging apparatus by which a specific nucleus density distribution of each tissue in a biological body is measured from outside the biological body without operating or cutting open the biological body, utilizing a magnetic resonance phenomenon.
2. Background Art
Recently, a great deal of the magnetic resonance imaging apparatus have been utilized while a medical diagnostic apparatus has been actively developed.
In a magnetic resonance imaging technique, chemically and physically macroscopic data on a molecule can be obtained by utilizing the fact that a nucleus of an atom absorbs an energy of a radiofrequency field resonantly when an intrinsic spin and its group of each magnetic moment accompanied thereby are placed under a uniform static magnetic field whose intensity is H.sub.o. The nucleus resonates, in a plane vertical to the direction of the static magnetic field, with an angular frequency .omega..sub.o shown in the following Larmor equation which is an equation defining the resonance condition including a relationship between the angular frequency .omega..sub.o, a gyromagnetic ratio .UPSILON. that is intrinsic to a type of of an atomic nucleus, and the static magnetic field strength H.sub.o. EQU .omega..sub.o =.UPSILON..multidot.H.sub.o
There are considered methods for imaging a spatial distribution of a specific atomic nucleus (for example, a hydrogen nucleus in water and fat) in the biological body utilizing the magnetic resonance imaging, such as a projection reconstruction method by Lauterbur, a Fourier method by Kumar, Welti and Ernst et al., a spin warp method, that is a modified one over the above mentioned, by Hutchinson et al.
On the other hand, as methods for imaging fluid (e.g. blood) flowing through the biological body, there are available and widely known a method using a flow encode pulse (phase contrast technique) by Moran, a time of flight method utilizing an inflow of nonsaturated fluid to an imaging region, and so on.
More recently, very noticeable is a method which images an increase of a local fresh blood accompanied by activation of a brain. Though the fresh blood contains much hemoglobin oxide that is diamagnetic substance, there are constantly much deoxyhemoglobin in a vein, so that a local magnetic field homogeneity is decreased. When the fresh blood is increased accompanied by the activation of the brain, a density of the hemoglobin oxide is increased so as to improve the local magnetic filed homogeneity. A change caused thereby can be captured by obtaining an image where T.sub.2 * (T.sub.2 star) is emphasized by an imaging method which is sensible to the change of the magnetic field homogeneity and in which the echo time is prolonged utilizing a gradient magnetic field echo method such as a field echo method for a long TE and a FID type EPI (echo-planar imaging), so that an activated portion is obtained as a bright area.
By these brain functional imaging methods, activation imaging is possible for a response of vision caused by an optical stimulus and a motion caused by finger movement. For example, by knowing which part of the brain is activated due to the finger movement, a brain cell controlling the finger movement can be specified.
However, recently there is reported a problem where a portion other than a cortex is captured as the bright area. This is because the blood containing a high-density hemoglobin oxide caused by the cortex activation flows into a somehow larger-sized vein existing in a downstream area, so as to cause the magnetic field inhomogeneity. Thereby, it becomes difficult to specify an activated cell.
In order to avoid the above problem, it is suggested that a diffusion of magnetization shall be observed instead of detecting a change of T.sub.2 * which is a transverse relaxation time affected by the magnetic inhomogeneity. However, the change is so little that a S/N ratio is not sufficiently good. Thus, a highly qualified image can not be obtained.
Moreover, another reason why the vein becomes bright is that upon activation of the brain, the flow rate of the vein is changed by some tens of percent, so that such a portion is imaged as the bright area specially in the field echo technique, due to a time-of-flight effect similar to an MR angiography. In order to suppress such an effect, it is considered to make use of a smaller flip angle of a radiofrequency (RF) pulse. However, when the flip angle of the RF pulse is made small, the S/N ratio is deteriorated. The time-of-flight effect can not be completely suppressed, even though there is a drawback where the S/N ratio is deteriorated as mentioned above.
In view of the above drawback, a travel motion of the vein is imaged beforehand by the MR angiography so that distinction between the cortex and the vein is facillitated.
However, in the above conventional technique, it is necessary to take the MR angiography as an extra imaging, so that the imaging duration is further prolonged.
Moreover, in view of T.sub.2 * imaging alone, the conventional T.sub.2 * imaging by a fast spin echo (FSE) method, a hybrid method or the like is considered advantageous in aspects of time resolution and singal-ro-noise ratio (SNR). However, in these sequences, the spin echo having no influence of magnetic inhomogeneity in principle is taken as central data at the time reconstructing the image. Thus, it is difficult to achieve a sufficiently sensible T.sub.2 * imaging.